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Here's a mystery Encyclopedia Brown probably couldn't solve: the case of the missing quasar. But astronomers appear to be up for the task. They're excited about a distant quasar that appears to have dimmed dramatically during the last decade, because it validates their understanding of these phenomena.

For a long time scientists were mystified by quasars, fairly compact objects in the sky that are extremely bright, in some cases ten or even 100 times brighter than the Milky Way Galaxy. Some scientists even speculated that these quasi-stellar objects were the other "side" of a black hole out of which all the material sucked in eventually emerged. By the 1980s, however, astronomers began to understand that quasars actually surrounded the very large, supermassive black holes at the center of galaxies. All of the electromagnetic energy quasars generate, they believed, comes from material falling into the black hole.

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About 13 years ago, scientists first measured the spectrum of a quasar known as SDSS J1011+5442. To estimate the amount of gas falling into its central black hole, they looked at its hydrogen-alpha emission line. When they looked at the same quasar again in 2015, they found that emission of this gas had fallen by a factor of 55. As a result of this unprecedented decline in hydrogen-alpha emissions, it has become known as the "changing-look quasar."

In the last year, astronomers have considered several possibilities to explain the rapid dimming. One is a passing dust cloud, but astronomers don't believe it could have moved quickly enough to cause such a significant dimming. Another possibility, a temporary flare caused by the black hole ingesting a nearby star, was also dismissed as unlikely.

That left a final explanation: the black hole had gobbled up all of the nearby gas for the time being.